Electrodeposition of chromium

ABSTRACT

The specification discloses an improved method and electrolyte solution for providing metal coatings on metals. These metals are electrolytically deposited from solutions comprising aprotic dipolar organic solvents which may contain water. The specific dipolar aprotic organic solvents disclosed are dimethylsulphoxide, dimethylacetamide, tetrahydrothiophen dioxide, dimethylformamide, propylene carbonate, tetramethyl urea and hexamethyl phosphoramide.

United States Patent 1 [111 3,772,170

harucha 1 Nov. 13, 1973 ELECTRODEPOSITION F CHROMIUM 902,755 11/190Meyer 204/14 N 6] n entor: Nanabhai Rusmmji Bharucha 4530 3,336,6588/1967 Husm 204/23 X Cote des Neige, Apt. 105, Montreal, OTHER CATIONS QCanada R. D. Blue et al., Trans. Electrochemical Soc., Vol. 22 Filed:Feb. 16, 1972 231-238 (1933)- Donald B. Keyes et al., Studies in theElectrodepo- PP 226,859 sition of Metals, U. of Ill. Bulletin, No. 206,p. 14,

Related US. Application Data (1930)- [63] Continuation of Ser. No.32,402, April 27, 1970, and

a continuation-in-part of Ser. No. 679,250, Oct. 30, Pnmary ExammerG'Kaplan 1967, abandoned Att0meyBrumbaugh, Graves, Donohue and RaymondForeign Application Priority Data Oct. 31, 1966 Great Britain 48,761/66[57] ABSTRACT The specification discloses an improved method and [52]US. Cl 204/51, 204/14 N, 204/45 R, electrolyte solution for providingmetal coatings on 204/48, 204/49, 204/ R, 204/52 R, metals. These metalsare electrolytically deposited 204/53, 204/54 R, 204/55 R from solutionscomprising aprotic dipolar organic sol- [51] Int. Cl C23b 5/06 ventswhich may contain water. The specific dipolar [58] Field of Search204/14 N, 51 aprotic organic solvents disclosed are dimethylsulphoxide,dimethylacetamide, tetrahydrothiophen diox- [56] References Cited ide,dimethylformamide, propylene carbonate, tetra- UNITED STATES PATENTSmethyl urea and hexamethyl phosphoramide.

3,131,134 4/1964 Micillo 204/14 N 5 Claims, No DrawingsELECTRODEPOSITION OF CHROMIUM This is a continuation of application Ser.No. 32,402 filed Apr. 27, 1970 which is turn was a continuation-inpartof application Ser. No. 679,250 filed Oct. 30, 1967, both applicationsnow abandoned.

The instant invention is directed to electroplating, i.e., theelectrodeposition of metal coatings and to electrolyte solutions forproviding coatings of such metals.

in recent times substantial improvements have been made in theappearance and performance of electrolytically deposited metal coatings.Particularly significant improvements have been achieved in theelectroplating industry. However, substantial problems and disadvantagesexist in the presently available processes for the preparation ofmetallic coatings, such as electrolytically deposited chromium coatings.

The general object of this invention is to provide an improved processfor the electrodeposition of metallic coatings, and to provide improvedplating baths for use in electroplating processes.

A particular object of the invention is to provide improved electrolytesolutions for the deposition of metallic coatings by electrolytictechniques.

A further object of the invention is to provide metallic coatings ofchromium and other metals which are characterized by improved corrosionresistance.

Another object of the invention is to provide a method for thedeposition of metallic coatings which is characterized by improvedcurrent efficiency and throwing power.

Another object of the invention is to provide a method for thedeposition of metallic coatings by electrolytic techniques characterizedby a reduction in the amount of hydrogen evolved at the cathode.

A still further object of the invention is to provide a method by whichsignificantly improved coatings can be deposited upon articles ofcomplex shape.

It is recognized that considerable economic advantage would be achievedif the current efficiency of metal plating techniques could beincreased. This is particularly so with respect to baths for theplatingof bright chromium. it is also recognized that the evolution of hydrogenfrom the cathode during an electrolytic deposition process is a seriousproblem and that coatings of improved corrosion resistance andappearance could be provided if the evolution of hydrogen could bereduced. In the past, experimentation with the use of modified chromicacid electrolyte solutions has shown little prospect of improvement inthe quality of the deposit without a concomitant reduction in currentefficiency.

in the past, basic studies in typical reactions, particularly inconnection with solutions containing trivalent chromium ions, havesuggested several possible methods for increasing current density,extending the plating current density range, and improving the generalappearance of the metallic coating. One such method, the addition ofbuffers to the plating bath to prevent or diminish a change in pHresulting from hydrogen evolution, produced only moderate improvementsin plating performance. Another attempt at improvement involved theaddition of organic complexing agents to the plating bath. Suchadditions were invariably associated with a substantial reduction incurrent efficiency.

Broadly, the instant invention is directed to the plating of metalswhich can be electroplated or deposited from solution such as chromium,iron, tin, manganese,

zinc, copper, lead, nickel, cobalt, and cadmium. Chromium is ofparticular interest in view of its desirable appearance and highintrinsic resistance to tarnish and corrosion in various indoor andoutdoor environments. it has been discovered that metallic deposits,particularly chromium deposits having greatly improved corrosionresistance properties and exhibiting very good appearance, may beproduced by electrolytic techniques from a plating bath containingchromium ions, preferably trivalent chromium ions, in which theelectrolyte comprises a dipolar aprotic organic solvent. in view of thebroad scope of the problems of hydrogen evolution at the cathode duringelectroplating processes, it is apparent that this development is ofgeneral application in and to the electroplating industry and is ofparticular advantage in connection with the plating of metals which aredifficult to plate because of their position in the standardelectromotive series.

The term dipolar aprotic organic solvent as used herein, including theappended claims, refers to an organic solvent which is capable ofdissolving a substantial amountof a salt of the metal to be depositedand which does not donate a substantial amount or quantity of hydrogenions. It is to be understood that the solvent must permit ionization ofthe dissolved metal salts in the bath. Preferred dipolar aprotic organicsolvents are those having high dielectric constants, e.g., greater thenabout 15, possess one or more electron donating groups, and are capableof solvating cations in the bath. Although these solvents may containhydrogen atoms they should not donate substantial quantities of labilehydrogen atoms to form strong hydrogen bonds or to be discharged at thecathode during electrolysis.

As examples of common dipolar aprotic solvents there may be mentioneddimethylformamide, dimethylacetamide, dimethylsulphoxide,tetrahydrothiophen dioxide, propylene carbonate, tetramethyl urea,hexamethylphosphoramide, and the like.

Preferred dipolar aprotic solvents are dimethylformamide anddimethylsulphoxide. Dimethylformamide has a relatively wide liquid rangehaving a boiling point at C., a high dielectric constant of about 37.5and a low vapor pressure at ordinary temperature. Dimethylformamide isalso miscible with water in all proportions and has a substantialability to dissolve chromic salts.

The dipolar aprotic organic solvents disclosed herein can be employed asthe sole solvent in the plating bath. Alternatively, the plating bathmay be formulated to contain water as a co-solvent. Generally, it ispreferred that the amount of water be limited to amounts of up to 40percent by weight, and it is particularly preferred to limit the amountof water present in the solution to a maximum of about 15 percent byweight.

It has been discovered that, in addition to functioning as a solvent,the dipolar aprotic solvent serves as a complexing agent. This uniquequality makes the addition of any other complexing agent to the platingbath totally unnecessary and, in fact, undesirable.

The conductivity of the dipolar aprotic organic solvent can beconveniently raised to any desired degree in order to permit the passageof suitable plating currents by the addition of water alone orcontaining inorganic ions other than those of the metal to be deposited.Sodium chloride is one such additive which will have this effect.

The metal to be deposited can be conveniently incorporated in theplating bath in the form of a metal salt which is soluble in the dipolaraprotic solvent, e.g., a halide, nitrate, acetate, formate, oxalate,sulfate alums, and the like. Chloride salts such as hexahydrated chromicchloride are preferred. In general, the solubility of the metal saltshould be sufficient to provide a metal ion concentration in the bath ofat least about 0.5 molar. Satisfactory operation of plating baths, asherein disclosed, has been obtained with a concentration of about Imolar with respect to chromic ions. In the case of partially aqueoussolutions, the pH of the aqueous portion of the solution can be adjustedbefore addition of the non-aqueous solvent.

The vessel for the plating bath can be of any suitable design and maypreferably be constructed of glass, ceramic, chemical stoneware, or anyother suitable nonconducting material.

The plating baths of this invention are superior to conventionalchromium plating baths in that they are characterized by enhancedcurrent efficiency and plat ing rate. They have been found to functionover a wider plating range, have greater covering power and improvedthrowing power. The chromium deposit obtained by the use of such bathshas been found to be substantially crack-free.

Table 1, below, shows plating conditions and results for a series oftests carried out in a Hull type cell without a porous diaphragm andusing an inert graphite anode. The cathode was nickel plated copper. Theupper and lower limits of current density over which a bright plate wasobtained were recorded and their ratio was recorded as the plating rangeratio. Dimethylformamide (DMF) was employed as the dipolar aproticsolvent, and the solution in each case was 1 molar CrCI TABLE I Temper-Total Plating Solution ature Cell Cell Range DMF pH "C Current \roltageRatio 10 2.2 40 3.0 40 7.2 30 2 l 42 3.0 40 7.0 50 2.0 45 3.5 42 8.0 702.3 55 4.0 38 6.0 90 2.3 56 4.5 70 7.0 95 L6 57 2.0 70 8.0 90 1.0 55 2.560 2.1 90 1.48 56 2.5 60 2.6 90 L4 55 2.5 6l 3.9 90 1.78 55 2.5 60 7.090 2.l 54 2.5 60 6.9 90 2.4 55 2.5 60 5.1

The invention also includes a method for the electrodeposition of ametal which comprises immersing an article which is to be cathodicallyplated in a plating bath containing ions of the metal to be depositedand an organic dipolar aprotic solvent, and electroplating the articleby passing an electric current through the plating bath to effectelectrodeposition of metal upon said cathode.

The operating conditions under which the plating of a metal upon acathode is achieved are, at least in part,

determined by the type of equipment employed, the nature of the metallicions to be deposited on the cathode, and the shape and composition ofthe cathode itself. Bath temperatures below the boiling point of thedipolar aprotic solvents are generally employed. Preferably suchtemperatures range from about F. to about 140 F. Plating is generallyaccomplished at a current density of from about I to about 20 andpreferably 1 to about 15 amperes per square decimeter. The pH of thesolution is generally maintained in the range from about 1 to about 2.5and preferably about 2.

An illustrative bath may comprise dimethylformamide and water in avolume ratio of about 4:1 containing about 1 mole of a metal salt perliter of solvent.

I claim:

I. A process for depositing a chromium coating on an article whichcomprises the steps of immersing said article as the cathode of anelectroplating apparatus into a plating solution comprising a dipolaraprotic organic solvent selected from the group consisting ofdimethylformamide, dimethylsulphoxide, dimethylacetamide, andtetrahydrothiophen dioxide, trivalent chromium ions, and a complexingagent which consists essentially of said dipolar aprotic organicsolvent, and passing an electric current through said plating solutionthereby to effect electrodeposition of the chromium onto said article.

2. The process of claim 1 wherein chromium metal is electrodepositedfrom a solution comprising about one mole of chromic chloridehexahydrate per liter of a solution comprising dimethylformamide andwater in a volume ratio of about 4:1.

3. An electrolyte solution for the electrodeposition of chromiumcomprising trivalent chromium ions, a dipolar aprotic organic solventand a complexing agent which consists essentially of said dipolaraprotic organic solvent, said solution consisting essentially of about Imole of chromic hexahydrate per liter of solvent and said solentcomprising dimethylformamide as complexing agent in water in a volumeratio of about 4:1.

4. A process for depositing a chromium coating on an article whichcomprises the steps of immersing said article as the cathode of anelectroplating apparatus into a plating solution comprising a dipolaraprotic organic solvent, trivalent chromium ions, a complexing agentwhich consists essentially of said dipolar aprotic organic solvent andfrom about It) percent to percent by weight water, and passing anelectric current through said plating solution thereby to effectelectrodeposition of the chromium onto said article.

5. An electrolyte solution for the electrodeposition of chromiumcomprising trivalent chromium ions, a dipolar aprotic organic solventand a complexing agent which consists essentially of said dipolaraprotic organic solvent and from about 10 percent to 90 percent byweight water.

izgigio j NI ED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 3,772,170 Dated November 13, 1973 Inventor(s) Nanabhai RustomjiBharucha It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 40, solen t" should be -solvent--.

Signed and sealed this 9th day of April 197A.

(SEAL) Attest:

EDWARD I'-i'.FLETCHER,JR. C. MARSHALL DAMN Attesting OfficerCommissioner of Patents

2. The process of claim 1 wherein chromium metal is electrodepositedfrom a solution comprising about one mole of chromic chloridehexahydrate per liter of a solution comprising dimethylformamide andwater in a volume ratio of about 4:1.
 3. An electrolyte solution for theelectrodeposition of chromium comprising trivalent chromium ions, adipolar aprotic organic solvent and a complexing agent which consistsessentially of said dipolar aprotic organic solvent, said solutionconsisting essentially of about 1 mole of chromic hexahydrate per literof solvent and said solent comprising dimethylformamide as complexingagent in water in a volume ratio of about 4:1.
 4. A process fordepositing a chromium coating on an article which comprises the steps ofimmersing said article as the cathode of an electroplating apparatusinto a plating solution comprising a dipolar aprotic organic solvent,trivalent chromium ions, a complexing agent which consists essentiallyof said dipolar aprotic organic solvent and from about 10 percent to 90percent by weight water, and passing an electric current through saidplating solution thereby to effect electrodeposition of the chromiumonto said article.
 5. An electrolyte solution for the electrodepositionof chromium comprising trivalent chromium ions, a dipolar aproticorganic solvent and a complexing agent which consists essentially ofsaid dipolar aprotic organic solvent and from about 10 percent to 90percent by weight water.